1. Field of the Invention
The present invention relates to a vibration isolation device which supports a structure such as a scanning electron microscope or the like, for example, and restrains its vibration.
2. Background
This type of conventional vibration isolation device is constituted so that an upper plate arranged horizontally is supported by coil springs above a lower plate fixedly arranged horizontally with a predetermined interval and a natural frequency is lowered by a viscoelastic body connecting the lower plate and the upper plate to each other, for example. And the coil springs improve vibration resistant effect in the vertical direction by increasing its deflection amount.
This device uses a vibration isolation theory that in case of vibration isolation of a structure, for example, a natural frequency (resonance frequency) of relating to vibration isolation depends on a deflection amount when supporting a structure, and that the larger the deflection amount is, the lower the natural frequency becomes, which improves the vibration resistant effect.
On the other hand, the Patent Document 1 discloses a vibration resistant device in which a lower plate is suspended by three or more links with respect to a fixed mount and these links are made to act as pendulums so as to lower a natural frequency in the horizontal direction and to restrain vibration in the horizontal direction.
Also, the Patent Document 2 discloses a vibration resistant device which adjusts a resonance frequency set in combination with a viscoelastic body while a natural frequency of a coil spring is kept lowered by pressing down a top part of the viscoelastic body so as to change its compression amount.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2000-161433
[Patent Document 2] Japanese Patent No. 3543758
However, the above-mentioned vibration isolation device and the vibration resistant device according to the Patent Documents 1 and 2 have the following problems.
That is, the above vibration isolation device is constructed to restrain vibration in the vertical direction and not constructed to restrain vibration in the horizontal direction. Therefore, it has been difficult to restrain the vibration in the horizontal direction.
On the other hand, in the vibration resistant device according to the Patent Document 1, since the links are used to restrain vibration in the horizontal direction, the structure becomes complicated.
Also, in the vibration resistant device according to the Patent Document 2, since the resonance frequency is adjusted at an installation site of the vibration resistant device, an installation work at the installation site becomes complicated.
The present invention was made in view of the above point one aspect is to provide a vibration isolation device which can effectively restrain vibration in the horizontal direction rather than in the vertical direction with a simple construction.
Other type of vibration isolation device has been required to put more emphasis on vibration isolation in the horizontal direction rather than in the vertical direction and is constructed as disclosed in Japanese Patent Laid-Open No. 2000-161433, for example, and shown in FIG. 17.
That is, in FIG. 17, a vibration isolation device 1 comprises a lower plate 2 fixedly arranged horizontally and an upper plate 3 arranged horizontally above it with a predetermined interval supported by 4 to 8 coil springs (lower springs 5 and upper springs 6) arranged in point symmetry through an intermediate plate 4 in the two-stage construction, and a viscoelastic body 7 connecting the lower plate and upper plate to each other in the vicinity of the center, by which a natural frequency is lowered.
Each of the lower springs 5 is stuck to the lower plate 2 at the lower end, while the upper end is stuck to the intermediate plate 4.
Also, each of the upper springs 6 is similarly stuck to the intermediate plate 4 at the lower end and the upper end is stuck to the upper plate 3.
Further, each of the upper springs 6 is arranged at a position with its elastic center aligned with the elastic center of the corresponding lower spring 5, respectively, so as to construct the two-stage coil spring construction.
Here, by selecting each of the coil springs 5, 6 with the free length longer than the outer diameter, a deflection amount in the vertical direction is selected smaller.
Moreover, the viscoelastic body 7 complements the small vibration damping performance of respective coil springs 5, 6 so that a sufficient vibration damping performance is kept and a natural frequency in the horizontal direction is a lower frequency.
When the vibration of an electronic microscope is to be isolated using so constructed vibration isolation device 1, the vibration isolation device 1 is arranged at four locations on a mount 8 made of steel members, and a lens barrel portion 9 which is particularly subjected to vibration in the electronic microscope is loaded on these vibration isolation devices 1 as shown in FIG. 18. By this, oscillation of the lens barrel portion 9 has a low frequency due to vibration isolation action of the respective vibration isolation devices 1, and vibration with a high frequency entering from a floor G to the mount can be reduced. [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-161433
Here, when a dummy load is loaded instead of the above-mentioned lens barrel portion 9 on the mount 8 shown in FIG. 18 using the above-mentioned vibration isolation device 1 and the vibration in the floor G and the dummy load portion are measured by a vibration sensor, it is known as shown in FIG. 19 that a resonance A by the mount 8 and the internal structure of the vibration isolation device 1, a resonance B by the dummy load and a natural vibration C by the vibration isolation device 1 are generated. These vibrations are divided into the natural vibration C (approximately 6 Hz) by the vibration isolation device 1, the resonance B by a machine mode of the dummy load itself and the resonance A considered to be caused by the mount 8 and the internal structure of the vibration isolation device 1.
By the way, according to a theoretical vibration isolation effect, the vibration with a frequency higher than an amplification area (around 6 Hz) in the vicinity of the natural frequency of the vibration isolation device 1 should be able to be reduced. However, the resonances A, B by the machine mode caused by the dummy load, the mount 8, and the internal structure of the vibration isolation device 1 should have been able to be vibration-isolated but actually, such a phenomenon occurs that these frequencies can not be vibration-isolated.
The present invention was made in view of the above and other points and considerations and can provide a vibration isolation device in which vibration of a frequency of a machine mode caused by the internal structure of the vibration isolation device is reduced with a simple construction.